Cold Process Soap Batch Size Calculator

How it works

Batter weight (g) ≈ mold volume (cm³) × 0.95; oils = batter ÷ (1 + water% + lye%); water = oils × water%; lye = oils × lye%; cured bar ≈ wet bar × 0.88

Soap recipes are built from the weight of oils, because the lye is calculated from the oils’ saponification values, but a mold is a volume — so the first question in designing a batch is how many grams of oil fill it. Raw soap batter weighs about 0.95 g per cubic centimetre, and it is made of oils plus roughly a third their weight of water and about 14% their weight of sodium hydroxide, so dividing the batter weight by that total fraction gives the oil weight to feed into a lye calculator. The old soaper’s shortcut of 0.4 times the mold’s cubic inches gives ounces of oil and comes from the same arithmetic. Bars lose 10–15% of their weight as water evaporates during the cure, so label weights should be based on cured bars, and fragrance is dosed on the oil weight.

Worked example

A 25.4 × 8.9 × 6.4 cm loaf holds 1,447 cm³ — about 1,374 g of batter. With 33% water and 14% lye that is 935 g of oils, 309 g of water and 131 g of NaOH, making ten 2.5 cm bars of about 137 g wet and 121 g (4.3 oz) cured, roughly 62¢ each in oils and lye.

Frequently asked questions